Many applications, particularly biomedical applications, require pressure measurements in very small places. For instance, measurements inside the cardiovascular systems of small mammals and even insects, within instrumented glaucoma valves, attached to miniaturized stents, and many others. Within the human cardiovascular system, there are arteries that are too narrow for conventional catheter-based pressure sensing techniques, or where the risk of introduction and removal is too great because of the large size of existing sensors. A recent, well-publicized example of heart surgery on an unborn fetus presents an interesting example case in which ultra-miniature pressure sensors would have allowed in-situ measurements of cardiac pressure during the procedure, greatly reducing the risk. Although this particular procedure was a success, many elements of this procedure could be improved by utilizing ultra-miniature pressure sensors or minimally-invasive probes to record signals.
Conventional micro-machined pressure sensors allow catheter-based measurements on adults and in many other situations. The largest of these sensors are several mm in cross-section, and are used for automotive applications (intake pressure, fuel tank pressure, etc.). A prior art sensor chip 100 is shown in FIG. 1A with a cross-sectional view thereof along line A-A′ shown in FIG. 1B. The sensor chip 100 comprises a silicon membrane 102 and piezoresistors 104. The chip size is 3 mm×3 mm and 300 μm thick. The width and length of each piezoresistors 104 are 15 μm and 150 μm, respectively. The piezoresistors 104 are mesa-etched and connected in a Wheatstone Bridge configuration. The membrane 102 has a depth of 250 μm defined by isotropic Reactive Ion Etching (RIE) of a silicon (Si) substrate. The cavity 106 is 1 mm in diameter and 50 μm in depth.
These micro-machined pressure sensors are also used for medical applications, such as for digital blood pressure instruments, and for measurements in which a fluid-filled catheter extends from the region where pressure is being studied to an external sensor. The use of a fluid-filled catheter to transmit the pressure allows use of accurate and bulky instrumentation, but suffers because of the distortion of the pressure signal during transmission. Mainly, this approach can transmit average pressure well, but the dynamic signals are distorted because of the propagation down the catheter.
All of these sensors are formed by etching cavities in a silicon wafer, bonding a second wafer to seal the cavity, forming piezoresistive strain gauges by ion implantation, and adding metallization to allow electrical contacts to external circuits. This approach has been commercially successful for many years and results in low-cost, high-performance sensors for many applications. The dimensions of these sensors are usually restricted to thickness and lateral dimensions greater than 0.3 mm. Partly, this is due to the wet chemical etching techniques used to controllably define membrane thicknesses, and partly this is due to the thickness of the starting silicon wafers.
Therefore, there is a need in the art for new methods and designs of pressure sensors and probes that overcome the limitations of currently available sensors and probes. More particularly, there is a need for pressure sensors on the sub-100 μm scale with the appropriate sensitivity and small enough to be placed on a probe with cross-sectional dimensions that are less than 100 μm.